The present invention relates to nuclear magnetic resonance (NMR) imaging and, more particularly, to methods and apparatus for combining the simultaneously received data from a plurality of radio-frequency (RF) coils of an NMR phased array in the time, rather than image, domain to produce a composite image having high signal-to-noise ratio (SNR) throughout the image.
The term "NMR phased array" refers to apparatus, such as shown in Roemer et al. U.S. Pat. No. 4,871,969 (the disclosure of which is incorporated herein by reference), wherein a plurality of closely-spaced RF coils is employed for simultaneously receiving different NMR response signals from associated portions of a sample (such as a patient in medical imaging) and combining the separate data from each coil to produce a single composite NMR image of the sample. By overlapping adjacent coils and connecting each coil to the input of an associated low-input-impedance preamplifier channel, the high SNR of a single surface coil can be maintained over fields-of-view (FOV) characteristic of remote coils.
Currently, composite images for NMR phased arrays are reconstructed in the image domain by combining the individual image contributions on a weighted, point-by-point basis after first acquiring the complete NMR images for each separate coil. The reason for acquiring the separate images first is that the optimum set of weights needed to maximize SNR when combining the separate signals to produce the composite image is a function of position, and so varies from point to point. While the phase shifters and transformers of the setup shown in FIG. 6 of Roemer et al. U.S. Pat. No. 4,825,162 can be adjusted to provide a composite image in the time domain having a high SNR at any particular point, different weighting must be applied for each point in order to obtain good sensitivity over the whole image. Thus, the conventional approach is to first separately acquire the different NMR image from each coil before combining the different individual images, on a point-by-point basis, to form the composite image.
NMR phased array imaging as described in the '162 patent, therefore, has the drawbacks of requiring large amounts of memory to store the separate coil images before reconstruction and of necessitating long time delays between acquisition of the last data point and onset of the first display of the reconstructed image.
It is desirable in NMR phased array imaging to be able to combine the data from the separate receiver coils as it is acquired on a time domain, rather than image domain, basis without sacrificing SNR resolution. Combining the data as acquired will reduce the total memory requirements of the system since only one combined data set would have to be stored and, because only the combined data set will have to be transformed at the end of scanning, will also reduce the time between end-of-scan and first appearance of the composite image.